A communications system permits at least one-way, and usually two-way, communications between a transmitter and a receiver. The transmitter may be part of a first transceiver and the receiver may be part of a second transceiver. In most communications systems, a power amplifier is used to increase the power level present in a signal to be transmitted over a channel from a lower power level to a higher power level to drive downstream components. In a system in which signals are transmitted by modulating a radio frequency, the power amplifier may drive an antenna.
The efficiency of a power amplifier determines the portion of power provided to a power amplifier that is included in the output of the power amplifier. The greater the efficiency of a power amplifier, the more effectively energy is converted into signal energy rather than being dissipated as heat. Furthermore, the greater the efficiency, the less power a device may consume to provide a desired level of output power. This is particularly important in battery operated devices where wasting power shortens the useful life of each charge on the battery.
A technique is disclosed in an article by Maloney, P. R. Upton, D. M. and McMorrow, R. J., entitled: "High Efficiency Communications Amplifier" published in Applied Microwave & Wireless, Summer 1994, that teaches the Doherty Amplifier Principle. The Doherty amplifier principle couples the output of two amplifiers directly into a single load impedance. The Doherty amplifier achieves high efficiency by operating one Class B amplifier into a load impedance two times larger than its optimum. This amplifier compresses and reaches peak efficiency at half of its maximum output power. A second amplifier is made active only during the peaks of the input signal and is used to modulate the effective load impedance presented to the first amplifier. Maximum efficiency is achieved when the second amplifier puts out full power. Thus, the first amplifier is kept on the verge of saturation for a 6 dB range of output power and near peak efficiency is maintained.
What is needed is an efficient power amplifier that more effectively converts energy into useful signal levels. Such an efficient power amplifier would provide a greater power output for a given input power and result in longer battery life for battery powered devices.